This invention relates to semiconductor switching devices, and more particularly, to high voltage semiconductor switching devices having reduced susceptibility to inadvertent device turn-on due to high voltage transients.
Thyristors, amplifying gate thyristors, triacs and transistors are semiconductor devices often used to turn-on and turn-off high voltage sources. These devices include at least first and second main current carrying electrodes and a gate electrode. A main voltage is applied across the first and second electrodes such that a main current flows therebetween upon application of a control signal to the gate electrode. The device is said to be in a turned-on state when conduction current flows between the first and second electrodes. Because the device has an internal junction capacitance, its forward-blocking capability is sensitive to the rate at which a forward voltage is applied to its main terminals. A steep rising voltage impressed across the main terminals may cause a capacitive charging current to flow through the device. The charging current (i=C.sub.j dv/dt) is a function of the inherent junction capacitive value and the rate of rise of the impressed voltage. If the rate of rise of impressed voltage exceeds a critical value, the capacitive charging current may be large enough to generate a gate current at a sufficient level and for a sufficient time to turn on the device. The ability of the device to withstand an impressed voltage transient across its main terminals is commonly termed the dv/dt capability specified in volts/microsecond. The dv/dt capability becomes of particular importance when voltage transients are impressed across the main terminals of the device. Voltage transients occur in electrical systems when some disturbance disrupts the normal operation of the system or even in normal circuit operation when other devices in the system switch on or off. Voltage transients generally have a fast rate of rise that may be greater than the dv/dt capability of the device. If the rate of rise of the transient exceeds the dv/dt capability of the thyristor, for example, it may cause the device to be inadvertently turned-on.
There are a number of known methods for increasing the dv/dt capability of the semiconductor device. One such method is the use of "emitter shorts" in a relatively large emitter area of a semiconductor device. Disadvantages that may occur with the use of emitter shorts are that the gate current required to activate the semiconductor device is increased and the di/dt rating of the device is also decreased.
Another known method of improving the dv/dt capability of the semiconductor device is the use of interdigitation. Interdigitation increases the initial turn-on area of the emitters and correspondingly reduces the turn-on sensitivity of the device to gate current. The interdigitation approach raises problems related to packaging as well as increasing gate current requirements.
A still further known method of increasing the dv/dt capability of the semiconductor device is the use of a resistor connected between the gate and the cathode of the semiconductor device, which provides a shunt path to divert a portion of the transient generated gate current away from the emitter of the cathode. The use of a resistive shunt path for the gate signal reduces the gate sensitivity of the semiconductor device as well.
The present invention concerns a high voltage semiconductor device in which the dv/dt capability of the device is increased by decreasing the amount of the transient capacitive charging currents conducted to the emitters of the device.
One object of the present invention is to provide a semiconductor device in which means are incorporated for increasing its dv/dt rating.
Another object of the present invention is to provide an improvement to the dv/dt rating of the semiconductor device without causing degradation of the other parameters of the device.
These and other objects of the invention will become apparent to those skilled in the art upon consideration of the following description of the invention.